What is this fabric tape made out of?

Question

Any ideas what is this fabric tape on my cool air return ductwork?

High res close-up

Some of it is separating or worse falling off of the duct work on the cool air return to my forced-air gas furnace. The furnace itself was installed in 1994; the house at some time has a coal-burning furnace, so I'm assuming this ductwork was installed during the retrofit from coal to gas. I'm not sure if this is the first or second gas furnace since the retrofit, but that at least tells us that the fabric tape was applied sometime before 1994.

It occurred to me that this fabric might have asbestos in it, which is a horrifying considering that it's all over the ductwork inside and out. For what it's worth, I was wearing a half-mask p100 respirator when I cleaning it up.

Answer 1

I would treat it as it is asbestos. Old coal heater is my clue. It really needs to be removed or encapsulated at least. It can be tested, but I bet the results would be positive for asbestos, remediation can be costly...